Using the Enable Pin with the LSF Family

Hello and welcome to The Logic Minute. In this video, the third installment in the LSF translator series, we will explain how to use the enable pin on the LSF family of devices.
When an LSF01 device is biased correctly, the enable pin is shorted to the Vref_B pin. And the highlighted voltage node is the gate bias for all channels of the LSF device. It's easier to understand this with numbers, so we've set up an example circuit with the B supply set to 3.3 volts and the A supply set to 1.8 volts. Recall from the bias video the bias voltage shows up across the external capacitor and is Vcca plus the bias FET's threshold voltage.
If a standard CMOS driver with a 1.8 volt supply is connected to the enable pin, the bias voltage is being forced to 1.8 volts, which is now 1 threshold value lower than what it should be and will cause problems with translation. This is the incorrect way to use the LSF's enable pin. The correct way to use the enable pin is with an open drain driver without a pull up resistor, as shown here.
When the open drain driver is outputting high impedance, then the Vref_B voltage node is allowed to naturally adjust to the correct bias value, as was explained in the bias circuit video. And when the open drain driver is forcing the Vref_B voltage node low, then all channels of the LSF device are put into a high impedance state, because the gate voltage is being forced to 0 volts. Note that the supply voltage of the open drain device can be completely different from the supplies used for the LSF and has no impact on the operation.
The LSF02 series of devices work differently than the LSF01 series. One of the big improvements was the simplification of the enable pin. In this case, the switch to open drain is done internal to the LSF02 device. And the external pin needs to be driven to either Va or ground.
For the LSF0204, the device is active for a high input at the enable pin and disabled for a low input to the enable pin. There is also an LSF0204D device that has an active low enable pin. Even though the enable pin for the LSF02 series is referenced to the A supply, it can be operated with the B supply for normal operation, because the input is overvoltage tolerant up to 7 volts.
This schematic shows the typical supply connections for an LSF02 series device. Please click on the links below to go to the video of interest. And thanks for watching.

Description

October 17, 2017

The enable pin of the LSF family of devices is not as intuitive as other devices in our portfolio. This video goes into details on the right way to use the enable pin.